Discrimination circuitry?

[SilverFinger]

I've been out of this great hobby for about 15 years until 2 weeks ago or so. I thought a new detector would out perform the old stuff I was used to, and I was flat out wrong. Other than being a whole lot lighter and better on battery life, it seems the same old rules of inductance still apply to modern discrimination circuits than in did 15+ years ago. Knock out the sinister pop tops and pull tabs and you still lose nickels and gold not to mention depth loss. I'm curious (because of a limited background in electronics) exactly what properties discrimination logic looks for when determining a metal type. You would think with all the new microprocessors, software, etc. the problem of junk metal discrimination would be a thing of the past. If I knew what I was doing, I'd study metals to assess certain properties of junk items by gathering data on their resistance, inductance, conductance, etc. versus valuable metals and program metal detector software to reliably identify (95% accuracy or so) that an item is junk. I guess what my question is is what makes a nickel or gold ring register the same as a pull tab, pop top, etc?

 

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[seas1to2]

a nickle an a pull tab don't register the same, they fall in the same range.if you have a new machine with a 2 or 3 digit # read out you can pick a nickle out from a pull tab with ease. you can do this beacuse a nickle is always the same size ,weight, but now you put in gold as in rings an now you got diffrent Karats, weight, shape etc. so they will all read diffrent.You can take a pull tab in perfect shape an get a reading on it, now give it a good bend, an the # will change 1 or 2 digits, or break part of it off an still a diffrent #, but it is still the same metal just diffrent shape or weight.I don't think you will ever get a machine to ID something as junk, to many possible good targets.

 

[Carl-NC]

I'm curious (because of a limited background in electronics) exactly what properties discrimination logic looks for when determining a metal type.

Primarily phase shift. The receive signal is compared to the transmit signal to see how much phase shift a target produces. This depends on the target's metal conductivity, but also size, shape, and orientation play a role. Gold and nickels happen to lie in the range of pulltabs and other trash. Newer detectors, especially those with VDI, do give you finer discrimination, making it possible to, say, correctly identify a square tab most of the time. Also, newer detectors have gotten a little better with depth, and better with ground tracking.

- Carl

 

[JOE(USA)]

back2thehunt,

The electronic theory hasn't changed, but the information on the target and the depth of the new machines sure have. The answers you have got are right on. There is a lot of good info here on Tnet by some very knowledgeable people (Carl-NC for one knows his stuff). Nothing wrong with using some of the vintage machines,I still sometimes use my old BH / Teknetics BB Pro and enjoy it very much. Glad to hear you got back into detecting and hope to hear more from you here on Tnet in the future. Joe